1. Field of the Invention.
The present invention relates to computer processing of fingerprint images
2. Description of the Prior Art.
Over the years, the most commonly used technique for obtaining fingerprints has been to apply ink to the tip of individual fingers and roll the inked fingertip at an appropriate location on an applicant card to produce the "rolled" fingerprint. Plain or "slap" prints, which are the simultaneous fingerprinting of the index, middle, ring, and little fingers of a hand, are taken by inking the tips of these fingers and simultaneous pressing the inked fingertips on the applicant card at the appropriate location. While these inking procedures will usually provide satisfactory images, they have their drawbacks. The inking procedure is messy. Several attempts are often required in order to obtain an acceptable fingerprint. Perhaps even a bigger drawback of this system is that the printed images are not easily adaptable to computerized storage and processing techniques, inhibiting cooperation and fingerprint data transfer between various police agencies.
Optical fingerprinting systems which optically generate fingerprint images are also in use. Several such optical fingerprinting systems are disclosed in the U.S. Pat. Nos. to Becker 3,482,498, McMahon 3,975,711, White 3,200,701, Schiller 4,544,267 and Marcus 4,553,387. However, for a variety of reasons, systems such as these have not gained widespread acceptance.
Due to the compound curved nature of the fingerprint on a finger, it is difficult to optically obtain an image corresponding to a rolled fingerprint. The Schiller U.S. Pat. No. 4,544,267 discloses an apparatus in which a finger pressed against a platen provides a fingerprint object which is scanned by an interrogating beam of collimated light that is linearly displaced across the platen thereby maintaining a constant angle between the interrogating light beam and the plane of the object being scanned. The Marcus U.S. Pat. No. 4,553,837 discloses finger processing apparatus which includes a cylindrical-segment platen which supports a finger. Optical scanning equipment scans the circumference of the platen in such a manner that the angle of incidence of a light beam on the fingerprint object remains constant. The Becker U.S. Pat. No. 3,482,498 discloses several embodiments of an optical apparatus for producing a rolled fingerprint image, both of which utilize a prism having a totally reflecting surface. The embodiment shown in FIG. 1a utilizes a plurality of prisms and light sources, and produces only an approximation of the ball and side ridges. The embodiment shown in FIGS. 2 and 3 utilize a mechanical system actuated by a rolling finger to move and position a light source.
While the fingerprinting systems disclosed in the Schiller and Marcus patents, and the second embodiment disclosed in the Becker patent, may be capable of optically providing a rolled fingerprint image, these systems are less than wholly desirable. Perhaps most important, it is not possible to review the image being captured in real-time to determine whether or not critical information required for classification is being captured. Furthermore, the mechanical aspects of these systems, are relatively complicated. As a result, maintaining focus during the time required to obtain the entire rolled fingerprint image can be difficult. Although the fingerprint image produced by the first embodiment of the invention disclosed in the Becker patent provides an image in real-time, this image only approximates the rolled fingerprint image.
Prisms such as those disclosed in the McMahon U.S. Pat. No. 3,975,711 and the White U.S. Pat. No. 3,200,701 utilize the optical principle of total internal reflection to produce a fingerprint image. As such, the "plane" of the fingerprint must be imaged at an observation angle which is not perpendicular to the plane. Vertical scale errors, or distortions of distances on the fingerprint image from their true distances along a Y-axis which is generally parallel to a longitudinal axis of the finger, are therefore inherent. When the surface of the prism on which the finger is inserted is grooved as illustrated in the McMahon patent, horizontal scale errors which are distance distortions on the fingerprint image from true distances along a X-axis generally perpendicular to the longitudinal axis of the finger on the fingerprint, are also inherent. Furthermore, curvature errors are also produced. As a result of the vertical and horizontal scale errors, and the curvature errors inherent in the use of a grooved total internal reflection prism such as that shown in McMahon, the fingerprint image provided thereby is severely distorted from a true rolled fingerprint of the same finger.
Prisms which have grooved finger receiving surfaces such as those disclosed in the McMahon patent will not provide optimum surface contact between the surface of a finger and therefore its fingerprint, and the prism. Portions of the fingerprint which it may be desired to obtain will therefore be lost. Illumination of the fingerprint through prisms such as that shown in the McMahon patent is often unequal, resulting in an image which has varying intensities throughout its area. Furthermore the contrast between ridges and valleys in the fingerprint image produced by these prisms is generally relatively low.
Many police departments including the FBI require both plain or slap fingerprint images and individual rolled fingerprint images as part of their standard fingerprinting process. Prior art optical fingerprinting systems, however, are incapable of optically generating both individual rolled fingerprints and slap fingerprint images.
It is evident that there is a continuing need for improved optical fingerprinting systems. A system having the capability of capturing both slap and rolled fingerprint images would be especially desirable. An operator should be able to easily interface with the system, and observe in real-time the quality of the fingerprint image before it is captured. A system which can capture fingerprints from fingers of varying sizes would also be useful.
It is also evident that there is room for improvement in the prisms utilized by optical fingerprinting systems. Grooves in these prisms should be contoured in a manner which permits optimum contact between the fingerprint and grooved surface. A prism which can capture slap fingerprint images is also needed. A prism which reduces horizontal and vertical scale errors, as well as curvature errors, would also be welcomed. Furthermore, a prism which produces a high contrast fingerprint image is also needed. Other techniques which can correct for vertical and horizontal scale errors, and curvature errors so as to produce an enhanced fingerprint image would also be desirable properties of an optical fingerprinting system.